Project Summary Neurodegenerative diseases are devastating age-related disorders that represent a tremendous disease burden worldwide. The need for effective therapies is increasingly urgent as the population ages. Most familial adult-onset neurodegenerative disorders are caused by dominantly-transmitted gene defects (e.g. C9ORF72 and SOD1 in ALS, HTT in Huntington's, ?-synuclein in Parkinson's). Thus, one approach toward primary therapy for such disorders is to suppress expression of the offending genes. Antisense oligonucleotides (ASOs) are a promising class of therapeutics for dominantly-inherited neurodegenerative disorders. One ASO has been approved to treat spinal muscular atrophy, and five others are in clinical trials for Huntington's, Alzheimer's disease, ALS, and frontotemporal dementia (FTD). Nevertheless, there are two key unmet needs in the field of ASO therapeutics which require urgent and focused investment. The first is that the phosphorothioate backbone used in most oligonucleotide drugs often causes toxicity when administered into the central nervous system. We have identified ways to mitigate this toxicity through changes in the backbone modification pattern. However, the current approaches increase susceptibility to nuclease digestion, which will reduce duration of effect. In this proposal we will develop novel mixed-backbone oligonucleotides that combine further increases in potency and decreases in toxicity with long duration of effect. The second key unmet need is that for many disease genes, successful therapeutic approaches would need to discriminate between the mutant and wild-type (healthy) alleles, silencing the mutant copy while leaving the wild-type copy intact. We will use both innovative assays and novel chemical modifications to improve the ability of ASOs to discriminate between these alleles. Applying these insights, we will advance drug candidates for two ALS and ALS-FTD genes (C9orf72 and profilin1) into extensive testing in animal models we have established of these diseases. We will examine the safety, efficacy and duration of effect of our advanced ASOs both at the molecular level and at the level of change in disease phenotype. In this proposal, our laboratories will combine innovative chemistry with deep expertise in neurology and disease-relevant mouse models. We aim to develop broadly applicable platform technology with a substantial improvement in therapeutic index relative to the ASOs currently in clinical development. Moreover, we will identify novel allele-selective candidate drug candidates for C9orf72- and profilin1-dependent ALS-FTD.